1. Field of the Invention
The present invention relates to an electromagnetic rotating machine, and particularly to an electromagnetic rotating machine which is preferred for a turbofan mounted in an airplane.
2. Description of the Related Art
In a conventional aircraft jet propeller, i.e. turbojet engine, the pressure of the air taken in from an inlet is increased by a compressor, thus obtained compressed air is intermixed with fuel, which is then combusted by a combustor and injected backwards as high-temperature and pressure gas, and propulsion is obtained by means of the reaction force produced by the injection. On the other hand, there is a turbofan engine and the like in which some of the breathed air is caused to bypass and pass through a fan only without being sent to the core engine that consists of a compressor, combustor, and turbine, the flow speed of the air is increased by means of the fan, and then the air is injected backwards, thereby obtaining propulsion. Particularly, the turbofan engines in which the amount of air sent to the fan is larger than that in core engines are also called “high bypass-ratio turbofan engine, ” which are used in regions of the speed lower than the sound speed, have high fuel consumption efficiency, have low emission since a large portion of the breathed air is not combusted, and are low noise since bypass airflow covers the combustion gas. For this reason, the high bypass-ratio turbofan engines respond to the needs of the age in terms of energy conservation and elimination of environmental destruction such as global warming and acid rain, and are used frequently as civil aircraft engines or cargo aircraft engines.
Further, while the industrial structures have been globalized or become borderless, demand for civil aircraft or cargo aircraft tends to grow, and at the same time the number of passengers and cargo transportation volume tend to increase, thus high thrust turbofan engines with a larger fan diameter for civil aircraft or cargo aircraft are needed. Motive power that produces big torque is required in order to drive a fan with a large diameter. Therefore, there is adopted a method of driving the fan by providing an external power unit, i.e. an electric motor, and using the torque of the electric motor. For the sake of reference, Japanese Patent Application Laid-Open No. H10-018860 discloses a method of obtaining driving force from an electric motor.
However, in order to produce big torque when driving a fan with an electric motor, a magnetic field system which generates a large magnetic field, and an electric generating system with large capacity which passes high current through an armature coil are required. Moreover, there is a problem that the weight of the engine section increases due to the iron-core structure of the armature.
Incidentally, if a lightweight electric motor with low power consumption is devised, the above problems are resolved. Generally, when current (I) is applied to an armature coil in a magnetic field (B) of an electric motor, electromagnetic force with the size and direction that are proportional to the vector product (I×B) between the current and the magnetic field is generated. Here, when the magnetic field is changed in time, induced current is applied to the armature coil based on Faraday's law of electromagnetic induction, and electromagnetic force with the size and direction that are proportional to the vector product (i×B) between the induced current (i) and the magnetic field (B) is generated by the interaction between induced current (i) and the magnetic field (B). The armature coil outputs motive power from the coaxial shaft while rotating by means of the electromagnetic force. However, in order to cause the armature coil to rotate in a fixed direction, the direction of the electromagnetic force has to be always constant. In other words, the direction of the vector obtained as a result of the vector product (i×B) between the induced current (i), which is applied to the armature coil, and the magnetic field (B) has to be constant at all times. In the case of a normal direct current motor, the direction of the magnetic field system is made constant by permanent magnet or electromagnet, and further the direction of armature current is made constant by a commutator, thus the electromagnetic force is always maintained in a fixed direction, and the armature is rotary driven in a fixed direction. Therefore, when applying induced current to the armature coil by changing the magnetic field in time, the induced current also is changed in time, thus there arises a problem that the direction of the induced current has to be appropriately controlled with respect to this changed magnetic field in order to make the direction of the electromagnetic force constant. On the other hand, in order to change the magnetic field in time, excitation current has to be supplied to the coil, but the problem is that the coil has to be excited by the lowest possible electrical power.